1. Field of the Invention
The present invention is directed generally to underground trolley vehicles of the type used in mining applications and more particularly to such vehicles propelled by D.C. motors.
2. Description of the Background of the Invention
A variety of vehicles are used in underground mining. Many of the vehicles are of the type which are used in conjunction with a trolley system. A trolley system for an underground mine, for example a coal mine, is comprised of an overhead, high voltage, D.C., noninsulated conductor which runs along the roof of the mine. A pair of parallel rails runs along the floor of the mine. The rails are at ground potential such that a load can be connected between the high voltage conductor and the rails. Thus, underground trolley systems are based upon the same principles as are above ground trolley systems.
Mining vehicles used in underground trolley systems are typically series wound D.C. motors having brushes. When the vehicle is started from a dead stop, a large resistance is in series with the motor. As speed increases, the large resistance is dropped out, in a stepped manner, so that at full speed all of the resistance is out of the circuit. Such motors and control systems are simple, rugged, and well suited for the harsh environment found in underground mines. There are, however, a number of problems associated with such motors. First, the motor brushes tend to wear out thus necessitating replacement. Although the brushes themselves are not expensive, the brushes are not easily accessible and usually require that the vehicle be taken out of service for at least one shift. In addition, the labor costs associated with brush replacement are high. Furthermore, the large resistance is stepped out through the use of contactors which rely on moving parts and are the source of current arcs as contact is broken. Because of the very nature of such devices, routine maintenance is accepted.
It is known that brush-type D.C. motors are capable of being dynamically braked. For example, if a vehicle is coasting, and control is shut off, the motor acts as a generator and the energy is dissipated into passive resistors. The energy produced and dissipated is proportional to the speed of the rotor. Thus, as the rotor slows down, the braking force is reduced, and the dynamic braking becomes less effective. Less effective braking means greater distances needed to stop the vehicle.
It is desirable to replace the bush-type D.C. motors with brushless D.C. motors thereby eliminating the downtime and maintenance associated with replacement of the motor brushes. It is also desirable to replace the contactors and relays with solid state electronics. It is further desirable to replace brush-type D.C. motors with brushless D.C. motors capable of regenerative braking which is independent of the speed of the rotor. However, the application of brushless D.C. motors to underground trolley applications has met with a number of problems.
An underground trolley system is typically operated on high voltage D.C. such as 240 volts. Power is drawn from the overhead conductor by use of a trolley pole. The contact between the trolley pole and the high voltage conductor is not continuous as the vehicle moves along the rails. For example, uneven rails may cause the trolley pole to bounce or otherwise lose contact with the high voltage conductor. When the vehicle passes over a switch, the application of power from the overhead conductor may be interrupted. Similarly, loss of the electrical ground, caused by dirt, debris, or generally poor contact between the vehicle's wheels and the rails, may also cause an interruption in the application of power from the overhead conductor.
Even when contact between the trolley pole and the overhead conductor is good, and the ground connection is good, the power available from the overhead conductor is very noisy and full of voltage spikes as a result of other equipment operating on the system. It has been reported that a 300 volt D.C. line can fluctuate between 180 volts and 370 volts thus representing a variation of -40 and +25 percent, respectively. Switching transients due to electromechanical contactors switching off inductive motor loads are reported to be approximately 1500-2000 volts on a 300 volt line and lasting from two or three to several hundred microseconds. Lastly, lightening strikes result in surge voltages of about five times nominal line voltage and last for approximately fifty microseconds. The combination of interruptions in the application of power coupled with a noisy power source having large voltage spikes is not conducive to the use of solid state electronics.
In addition to power supply problems, the mine environment itself is extremely harsh. The relative humidity can be as high as 98 percent. Dust concentrations are typically 3 mg per cubic meter but may be a high as 10 mg per cubic meter Additionally, the vehicles are subjected to constant and substantial vibrations. The combination of humidity, dirt, and vibration is a substantial impediment to the implementation of solid state electronic drive systems.
During the 1970's there was substantial interest in using silicon controlled rectifiers (SCR) in solid state D.C. motor controllers for underground mining machines. The harsh environment, however, posed an unexpected problem. Due to the presence of methane, the SCR's had to be placed in explosion proof housings. At that time, all existing commercial SCR motor controllers used direct convection type cooling which was incompatible with the explosion proof housings. To overcome that cooling problem would have required substantial and expensive changes in machine design. Thus, the need still exists for a solid state D.C. motor controller which can be used on existing machines.